Rebuild when any included header file changes

[adetaylor]

We should ensure we output cargo:rerun-if-changed=src/input.h for each header file which is transitively included. We can do this using https://docs.rs/bindgen/0.56.0/bindgen/struct.CargoCallbacks.html but we probably want a more generic interface such that gen/cmd can do something sensible (e.g. generate a .d file) as well as gen/build.